1. Field of the Invention
The present invention relates to a display device, and more particularly to a COG packaging type liquid crystal display comprising an IC chip for driving a thin film transistor (TFT) on a glass substrate.
2. Description of the Related Art
The liquid crystal display device is comprised of a liquid crystal display panel which is further comprised of two substrates between which the liquid crystal layer is held, and a surface light source unit which is installed on the rear face side of the liquid crystal display panel. The liquid crystal display panel is normally constructed by two insulation transparent substrates made of glass, which face each other, where display material, such as liquid crystals, is sandwiched, so that voltage is applied selectively to this display material. One of the substrates is a thin film transistor array substrate (hereafter called TFT array substrate) where switching elements, such as thin film transistors (TFT) and pixel electrodes connecting these switching elements are formed in a matrix. The other substrate is a color filter substrate (CF substrate) which further comprises R, G and B color layers provided corresponding to the pixel electrodes, and a black matrix (BM) provided between these color layers.
In the TFT array substrate, source lines and gate lines are crossed via an insulation film to apply signals to the switching elements. A plurality of source lines and gate lines are disposed respectively, corresponding to the number of pixel electrodes. A COG type liquid crystal device, where the IC chip for driving the pixel electrodes is directly mounted on the substrate, is known (e.g. Japanese Unexamined Patent Application Publication No. 2000-347206, 2000-81635 and 2001-42282). This IC chip for driving is mounted at the end portion of the glass substrate, outside the display area of the substrate, via the anisotropic conductive film (ACF). And FPC is connected to the end portion of the glass substrate, and power and signals are supplied to the IC for driving via the lines provided on the glass substrate. The bump configuration of the COG packaging type IC for driving will be described with reference to FIG. 14. FIG. 14 is a top view depicting the configuration around the source driver IC, which is an IC for driving. The driver IC 101 is provided near the end portion of the glass substrate 27. At the longer side of the driver IC 101, a bump for output 16 is provided, and at the other longer side bumps for input are provided. The bump for output 16 is provided at the display area 34 side on the glass substrate, and the bumps for input are provided at the end portion of the substrate. The bumps for input are GND 1, bump for analog power supply 2, bump for digital power supply 3, bump for reference voltage at the positive polarity side 4 and bump for reference voltage at the negative polarity side 5. A plurality of the driver ICs 101 are disposed on the glass substrate, outside the display area, and FPCs (Flexible Printed Circuit) 21 are connected to the end portion of the substrate corresponding to the respective driver IC 101. On the side of the driver IC 101, cascade line is formed, by which a plurality of driver ICs are sequentially connected. This configuration, however, has the following problems.
Because of the dispersion of the ACF connections between the bumps for input of the driver IC 101 and the lines on the glass substrate, the resistance value tends to increase. Also many bumps for connection must be provided on the driver IC, so the bump configuration is restricted and the pitch between bumps cannot be freely increased. Therefore the pitch of the signals of the FPC 21 becomes large with respect to the pitch of the bumps for input of the driver IC 101, lines from the FPC 21 to the driver IC 101 become thin, and the line resistance value increases. By such an increase in the resistance between bumps and the FPC, the driver IC 101 may not operate normally or the desired voltage may not be output. And the operation of the driver IC may have problems, which decreases the display quality. Also in the case of a COG packaging type, the screen border size increases.
In this way, in the case of a conventional COG packaging type liquid crystal display device, the screen border size becomes big and the display quality drops if FPC to the driver IC is wired on the glass substrate.